High brightness LED package with compound optial element(s)

ABSTRACT

A light source includes an LED die with an emitting surface and a collimating optical element. The optical element includes an input surface in optical contact with the LED emitting surface, and an output surface. The optical element has a first portion that comprises the input surface, made of a first optical material, and a second portion that comprises the output surface, made of a second optical material. The first optical material, which may include sapphire, diamond, or silicon carbide, has a higher refractive index, thermal conductivity, or both relative to the second optical material.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application relates generally to the following co-filed andcommonly assigned U.S. patent applications: “High Brightness LEDPackage”, Attorney Docket No. 60217US002; “High Brightness LED PackageWith Multiple Optical Elements”, Attorney Docket No. 60219US002.

FIELD OF THE INVENTION

The present invention relates to solid state light sources, and hasparticular applicability in the field of packaged light emitting diodes(LEDs).

BACKGROUND

LEDs are a desirable choice of light source in part because of theirrelatively small size, low power/current requirements, high speed, longlife, robust packaging, variety of available output wavelengths, andcompatibility with modern circuit boards. These characteristics may helpexplain their widespread use over the past few decades in a multitude ofdifferent end use applications. Improvements to LEDs continue to be madein the areas of efficiency, brightness, and output wavelength, furtherenlarging the scope of potential end-use applications.

LEDs are typically sold in a packaged form that includes an LED die orchip mounted on a metal header. The header has a reflective cup in whichthe LED die is mounted, and electrical leads connected to the LED die.The package further includes a molded transparent resin thatencapsulates the LED die. The encapsulating resin typically has anominally hemispherical front surface to partially collimate lightemitted from the LED die.

BRIEF SUMMARY

The present application discloses packaged solid state light sourcesthat utilize LED dies. The LED die has at least one emitting surfacethat is in optical contact with at least one collimating optical elementsuch as a tapered element or lens element. The optical elements orelement has an input surface and an output surface, and is divided intoat least a first and second portion, where the first portion comprisesthe input surface and the second portion comprises the output surface.The first and second portions comprise a first and second opticalmaterial, respectively, where the first optical material has arefractive index and/or thermal conductivity greater than that of thesecond optical material. The first optical material can comprise aphysically hard, transparent inorganic material, such as sapphire,diamond, or silicon carbide.

These and other aspects of the invention will be apparent from thedetailed description below. In no event, however, should the abovesummaries be construed as limitations on the claimed subject matter,which subject matter is defined solely by the attached claims, as may beamended during prosecution.

BRIEF DESCRIPTION OF THE DRAWINGS

Throughout the specification, reference is made to the appendeddrawings, where like reference numerals designate like elements, andwherein:

FIGS. 1 and 2 are schematic sectional views of LED packages having abrightness enhancing layer;

FIGS. 3 and 4 are schematic sectional views of more LED packages havingbrightness enhancing layers, and tapered optical elements;

FIG. 5 is a graph showing modeled brightness and luminous output of anLED die as a function of the footprint size of the tapered element onthe front emitting surface of the LED die;

FIGS. 6, 7, and 8 are schematic sectional views showing LED packagesutilizing compound taper elements, and wherein FIG. 8 further showsmultiple taper elements coupled to an LED die; and

FIG. 9 is a schematic sectional view of another LED package having abrightness enhancing layer and multiple optical elements.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

One disadvantage of conventional LED packages described in theBackground section above is the inefficiency in getting the light thatis generated within the LED die transmitted to the outside environment,typically air. A major reason for this inefficiency is the highrefractive index of the semiconductor layers of the LED die, and thelarge mismatch in refractive index between the encapsulating resin andthe outer portion (defining the emitting surface) of the LED die. Thismismatch promotes total internal reflection (TIR) of much of the lightwithin the LED die, causing such light to become trapped and eventuallyabsorbed.

Another disadvantage of the typical LED package relates to poor heatmanagement of the LED die, which unduly limits the amount of currentthat can flow through the diode junction of the LED. This in turn limitsthe achievable brightness and luminous output of the LED package. Poorheat management, which refers to non-optimal heat removal from the LEDdie, also can adversely impact LED lifetime by causing the LED die torun hotter at a given current than desired. In the known LED packagedescribed above, bonding of the LED die to the metal header providesreasonable heat removal from the back of the LED die. However, the frontemitting surface of the LED die contacts the encapsulating resin, whichhas a low thermal conductivity and thus removes a minimal heat from theLED die.

It would be desirable for many end-use applications to provide LEDpackage improvements that could couple more of the light generatedwithin the LED die to the outside environment, thus enhancing theluminous output of the device. It would also be desirable to provide LEDpackage improvements that could enhance the brightness of an LED die (ata given drive current). It would also be desirable to provide LEDpackage improvements that could enhance the thermal management of theLED die to provide cooler LED die operational temperatures and/or higherachievable LED drive currents.

In the text that follows, LED packages that use at least one compoundoptical element in optical contact with the LED die, and advantagesassociated therewith, are disclosed amidst a discussion of relatedembodiments that may not require compound optical elements. This is sothat the reader will more fully appreciate design details and variationsof the claimed invention.

In FIG. 1, an LED package 10 includes an LED die 12 mounted on a headeror other mount 14. The die and mount are depicted generically forsimplicity, but the reader will understand that they can includeconventional design features as are known in the art. For example, theLED die 12 can include distinct p- and n-doped semiconductor layers,substrate layers, buffer layers, and superstrate layers. The primaryemitting surface 12 a, bottom surface 12 b, and side surfaces 12 c ofthe LED die are shown in a simple rectangular arrangement, but otherknown configurations are also contemplated, e.g., angled side surfacesforming an inverted truncated pyramid shape. Electrical contacts to theLED die are also not shown for simplicity, but can be provided on any ofthe surfaces of the die as is known. In exemplary embodiments the diehas two contacts both disposed at the bottom surface 12 b of the die,such as is the case with “flip chip” LED die designs. Further, mount 14can serve as a support substrate, electrical contact, heat sink, and/orreflector cup.

LED package 10 also includes a transparent optical element 16 thatencapsulates or surrounds the die 12. The optical element 16 has arefractive index intermediate that of the LED die (more precisely, theouter portion of the LED die proximate emitting surface 12 a) and thesurrounding medium, which is ordinarily air. In many embodiments it isdesirable to select a material for element 16 whose refractive index isas high as possible but without substantially exceeding the refractiveindex of the LED die, since the smaller the difference in refractiveindex between the LED die and the element 16, the less light is trappedand lost within the die. Optical element 16 as shown has a curved outputsurface, which can help ensure that light is transmitted out of the LEDpackage to the surrounding medium, and can also be used to focus orcollimate, at least partially, light emitted by the LED die. Opticalelements having other shapes can also be used to collimate light,including tapered shapes discussed further below.

LED package 10 is further provided with a patterned low refractive indexlayer 18 between the optical element 16 and the die, which has theeffect of selectively preserving some light entrapment in the LED die inorder to enhance the brightness in a localized aperture or area 20 atthe emitting surface 12 a. Patterned low index layer 18 is insubstantial optical contact with side surfaces 12 c and the portion ofemitting surface 12 a exclusive of aperture 20, while the opticalelement 16 is in optical contact with the portion of emitting surface 12a over the area of the aperture 20. (In this regard, “optical contact”refers to the surfaces or media being spaced close enough together,including but not limited to being in direct physical contact, that therefractive index properties of the low index layer or transparentelement, for example, control or substantially influence total internalreflection of at least some light propagating within the LED die.)Patterned low index layer 18 has a refractive index substantially lowerthan both the refractive index of the LED die and the refractive indexof transparent element 16. Layer 18 is also optically thick in thoseplaces where it is intended to promote light trapping. By opticallythick, we mean that its thickness is great enough to avoid frustratedtotal internal reflection, or that the refractive index properties ofthe medium on one side of the layer (such as the optical element 16) donot control or substantially influence total internal reflection of atleast some light propagating in the medium on the other side of thelayer (such as the LED 12). Preferably, the thickness of the patternedlow index layer is greater than about one-tenth, more preferablyone-half, more preferably about one wavelength for the energy of lightof interest in vacuum. By “patterning” of layer 18 we also mean toencompass embodiments where layer 18 is continuous over the LED emittingsurface, but made to be extremely thin (hence ineffective to maintaintotal internal reflection) in the aperture 20 and optically thickelsewhere. It is advantageous for layer 18 to be a transparentdielectric material, or to at least comprise a layer of such a materialat the surface of the LED die. These materials have advantages overreflective coatings made by simply applying a layer of metal to the LED,for example, because dielectric materials can provide 100% reflection(by TIR) for much of the light within the LED die, while simple metalcoatings have substantially less than 100% reflectivity, particularly athigh incidence angles.

Patterned low index layer 18 enhances the brightness of some portions ofthe LED (e.g., in the aperture 20) at the expense of reducing thebrightness of other portions of the LED (e.g., the portions of emittingsurface 12 a beyond aperture 20). This effect relies on the LED diehaving low enough internal losses during operation to support multiplebounce reflections of the emitted light within the LED die. As advancesare made in LED die fabrication and design, losses from surface orvolumetric absorption can be expected to decrease, internal quantumefficiency can be expected to increase, and brightness-enhancing effectdescribed herein can be expected to provide steadily increasingbenefits. Bulk absorption can be reduced by improving substrates andepitaxial deposition processes. Surface absorption can be reduced byimproved back reflectors such as by bonding the epitaxial layer to highreflectivity metal mirrors or by incorporating omnidirectional mirrorsin the LED structure. Such designs may be more effective when combinedwith shaping the backside of the LED die to increase light outputthrough the top surface. In exemplary embodiments, the majority of thebottom surface 12 b is a highly reflective material such as a metal or adielectric stack. Preferably the reflector has greater than 90%reflectivity, more preferably 95%, most preferably 99% reflectivity atthe LED emission wavelength.

Referring again to FIG. 1, an arbitrary emitting point source 22, forexample, emits light ray 24. The refractive indices of LED die 12 andtransparent element 16 are such that the ray on its first encounter withthe emitting surface 12 a at the LED/optical element interface would betransmitted into and refracted by element 16. Patterned layer 18,however, changes the interface at that location to be totally internallyreflecting for ray 24. The ray travels through the thickness of the LEDdie, reflects off the back surface 12 b, and again encounters theemitting surface 12 a, this time escaping into transparent element 16because of the absence of layer 18 as shown in FIG. 1. The portion ofemitting surface 12 a at aperture 20 is thus made brighter (moreluminous flux per unit area and per unit solid angle) at the expense ofthe portion of emitting surface 12 a covered by the low index layer 18.

In the embodiment of FIG. 1, some light within the LED that strikes thelow index layer 18 can still escape into element 16, if its angle ofincidence relative to the emitting surface 12 a normal vector issufficiently small so that it simply passes through low index layer 18.Thus, light striking the low index coated portion of the LED die willhave a non-zero but smaller range of escape angles than the uncoatedportions. In alternative embodiments, the low index layer 18 can beovercoated with a good normal-incidence reflector such as a reflectivemetal or an interference reflector to increase recycling of light in theLED die and further enhance the brightness at aperture 20, withoutlosing the benefit of TIR provided by low index layer 18. Optionally, aninterference reflector can be positioned between the outer die surfaceand the low index layer 18.

Suitable low index layers 18 include coatings of magnesium fluoride,calcium fluoride, silica, sol gels, fluorocarbons, and silicones.Aerogel materials are also suitable, as they can achieve extremely loweffective refractive indices of about 1.2 or less, or even about 1.1 orless. Aerogels are made by high temperature and pressure critical pointdrying of a gel composed of colloidal silica structural units filledwith solvents. The resulting material is an underdense, microporousmedia. Exemplary thicknesses for the low index layer 18 are from about50 to 100,000 nm, preferably from about 200 to 2000 nm, depending on therefractive index of the material. The refractive index of layer 18 isbelow the refractive index of the optical element 16, which can be amolded resin or other encapsulant material, and below the refractiveindex of the LED die, or that portion of the die proximate the emittingsurface(s). Preferably the refractive index of layer 18 is less thanabout 1.5, more preferably less than 1.4. Low index layer 18 can be asolid layer of dielectric material, or a vacuum or gas-filled gapbetween the LED die and transparent element 16.

The outer surfaces of the LED die can be optically smooth, i.e., havinga surface finish R_(A) of less than about 20 nm. Some, all, or portionsof the outer LED surfaces may also be optically rough, i.e., having asurface finish R_(A) greater than about 20 nm. Portions of the edges orthe top surface can also be at non-orthogonal angles relative to thebase of the LED die. These angles can range from 0-45 degrees fromorthogonality. Further, major or minor surfaces of the LED die need notbe flat. For example, a raised portion or portions of the emittingsurface of the LED die can contact a generally flat bottom surface ofthe optical element to define at least the apertures 20, 20 a, and 34 inFIGS. 1-3.

The shape of aperture 20, defined by the substantial absence of the lowindex layer 18, can be circular, rectangular, square, or more complexshapes, whether polygonal or non-polygonal, regular or irregular.Multiple apertures are also contemplated, as discussed in more detailbelow. The aperture shape(s) will typically be selected as a function ofthe intended application, and can be tailored to optimize the overallsystem performance. It is also contemplated to pattern the surface ofthe aperture with a continuous or discontinuous pattern or network oflow index coated areas, or provide the low index layer with a gradientin thickness or refractive index or both to modify the distribution oflight output over the surface of the aperture. The aperture can alsocover the entire top emitting surface 12 a, where at least portions ofthe side surfaces 12 c are covered with low refractive index layers.

Turning to FIG. 2, an LED package 10 a is shown there similar to LEDpackage 10, but where low index layer 18 has been modified by includinga network of low index coated areas within the central aperture. Themodified low index layer is thus labeled 18 a, and the modified centralaperture is labeled 20 a. Other elements retain the reference numbersused in FIG. 1. As shown, the network of low index areas can be arrangedin a pattern that is relatively dense near the edges of the aperture sothat transmission is relatively low in that region. The ability totailor the transmission through the aperture is useful in highbrightness LEDs where a specific spatial uniformity or outputdistribution is required for the system design. Such an arrangement oflow refractive index medium within an aperture can likewise be appliedto other disclosed embodiments, including without limitation theembodiments of FIGS. 3, 4, and 6-8.

The aperture can be coated with a low index material having a differentthickness or different refractive index or both relative to the lowindex material defining the aperture (referred to as the “surroundinglow index material” for convenience). Such design flexibility can beused to modify the angular distribution of light emitted by the packagedLED. For example, coating the aperture 20 or 20 a with a material thathas a refractive index between that of the optical element 16 and thesurrounding low index material will restrict the range of angles oflight emitted by the aperture. This will cause light that wouldordinarily be emitted at high angles to be recycled within the LED die,and increase the output of light in a range of angles that can be moreefficiently used by the associated optical system. For example,collection optics used in electronic projection systems do notefficiently use light that is outside the commonly used F/2 to F/2.5acceptance design angles.

Turning now to FIG. 3, an LED package 30 includes a transparent opticalelement 32 in partial optical contact with LED die 12 and partiallyspaced apart from the LED die to define a substantial air gap 34therebetween. Transparent element 32 has an input surface 32 a and anoutput surface 32 b, the input surface 32 a being: smaller than outputsurface 32 b; smaller than emitting surface 12 a of the LED die; and inoptical contact with a portion of the emitting surface to defineaperture 34. In this regard, the input surface is “smaller” than theoutput surface because it has a smaller surface area, and the outputsurface is accordingly larger than the input surface because it has alarger surface area. The difference in shape between the optical element32 and the emitting surface 12 a produces an air gap 36 which forms apatterned low refractive index layer around the area of contact(aperture 34). Light generated by the LED die can thus be efficientlyextracted at the aperture 34 by the transparent element 32 with a highbrightness. The optical element 32, and other optical elements disclosedherein, can be bonded to the LED die at the point of contact by anysuitable means, or it can be held in position without being bonded tothe LED die emitting surface. Further discussion regarding non-bondedoptical elements in LED packages can be found in co-filed and commonlyassigned U.S. patent application “LED Package With Non-Bonded OpticalElement” (Attorney Docket No. 60216US002), which is incorporated hereinby reference in its entirety. As discussed above, the range of angles oflight emitted by the LED emitting surface 12 a into optical element 32over the aperture 34 can be reduced by interposing a layer of materialwhose refractive index is between that of the LED die 12 and transparentelement 32.

Another approach for reducing the range of angles of collected light—orfor collimating (at least partially) the collected light—is to use atransparent element having one or more tapered side walls, as shown inFIG. 4. There, LED package 40 is similar to LED package 30, but opticalelement 42 is substituted for optical element 32. Element 42 has aninput surface 42 a and an output surface 42 b, the input surface 42 abeing: smaller than output surface 42 b; smaller than emitting surface12 a of the LED die; and in optical contact with a portion of theemitting surface to define aperture 44. The difference in shape betweenthe optical element 42 and the emitting surface 12 a produces an air gap46 which forms a patterned low refractive index layer around the area ofcontact (aperture 44). Furthermore, optical element 42 includes taperedside surfaces 42 c, 42 d, which are reflective in order to collimatesome of the highly oblique light entering input surface 42 a from theLED die. Reflectivity of the side surfaces 42 c, 42 d can be provided bya low refractive index medium that supports TIR, or by application of areflective material such as a metal layer or interference reflector, orcombinations thereof.

The optical element 42 can be in optical contact with the emittingsurface of the LED die through fluids, thermally bound inorganicglasses, plastic inorganic glasses, or by providing the surfaces with anoptically smooth finish (surface roughness R_(A) less than about 50 nm,preferably less than about 20 nm) and then holding the surfaces in closeproximity to each other. Furthermore, optical element 42 can be compoundin structure, where the lower tapered portion comprising surfaces 42 a,42 c, 42 d is made separately from the upper lens-shaped portioncomprising surface 42 b, and the two portions adhered or otherwisejoined together by conventional means. The broken line is provided toshow the two portions more clearly. More discussion of compound opticalelements, design considerations, and associated benefits is providedbelow.

A model was used to determine the potential increase in brightness for apackaged LED that utilized a patterned low index layer and a taperedoptical element coupled to the output aperture. An LED was modeled withthe material properties of silicon carbide (index 1.55) having anemitting region, an absorptive region, and angled edge facets such as torepresent the optical behavior of a typical LED. An inverted truncatedpyramid-shaped tapered optical element was optically coupled to thefront facet or emitting surface of the LED. The material properties ofthe optical element were those of silicon carbide. The LED had a squareshape as viewed from the front, as did the input and output surfaces ofthe optical element. The model further coupled the output surface of theoptical element to a half-sphere lens with the material properties ofBK7 glass, where the diameter of the lens was ten times the width of thesquare LED emitting surface, and the radius of curvature of the lens wasfive times the width of the LED emitting surface. The size of the inputsurface of the optical element, was incrementally changed from 100% ofthe LED emitting area to 4%, while keeping the aspect ratio of theheight of the optical element 2.2 times the width of the output surfaceof the optical element, and keeping the width of the output surface 2times the width of the input surface. As the size of the optical elementbecame less than the size of the LED emitting surface, a medium ofrefractive index of 1 was assumed to cover the portion of the LEDemitting surface outside of the optical element input surface, thusforming a low refractive index patterned layer that covered the LEDemitting surface in complementary fashion to the optical element inputsurface. The fractional power emitted by the optical element(representative of the relative luminous output of the LED package) andthe relative irradiance (lumen/(cm²sr) emitted by the output surface ofthe optical element (representative of the relative brightness of theLED package) was calculated. FIG. 5 depicts in a general way the trendobserved. Curve 50 is the relative fractional power emitted; curve 52 isthe relative irradiance. The results confirm that as the aperture sizedecreases, less total luminous output is obtained from the package, butthe brightness (in the smaller aperture) can increase dramatically.

The patterned low index layer of disclosed embodiments can comprise agap or a coating of low index material applied to the LED die. Suitablemethods for coating the LED die with a low index material—or withindividual layers that will form an interference reflector—from a liquidinclude spin coating, spray coating, dip coating, and dispensing thecoating onto the die. Liquid coatings can be composed of monomers thatare subsequently cured, solvents, and polymers, inorganic glass formingmaterials, sol gels, and Aerogels. Suitable methods of coating the lowindex material from a gas state include chemical vapor deposition orcondensing a vapor on the die. The die can also be coated with a lowindex material by sputtering, vapor deposition, or other conventionalphysical vapor deposition methods.

The coatings can be applied to a multitude of LEDs at the wafer level(before dicing), or after the wafer is diced but before mounting, afterthe die is mounted on the header or other support, and after electricalconnections are made to the die. The aperture can be formed before orafter the low index coating is applied. The choice of post-coatingpatterning method may depend on the particular low index material(s)chosen, and its compatibility with semiconductor processing. Forexample, a wafer can be covered with photoresist and patterned to createopenings where the apertures are desired, a suitable low index coatingdeposited, and then liftoff performed using suitable solvent.Alternatively, a low index material can be deposited first over theentire wafer or die, a patterned photoresist layer can be applied as anetch mask, and the low index material removed using a suitable techniquesuch as reactive ion etching. The photoresist layer can optionally bestripped using a suitable solvent. Other techniques for patterning thelow index material include laser ablation and shadow masking, which maybe particularly useful with materials that are soluble in typicalphotolithography stripping or development solvents. Suitable methods forlifting the unwanted coating off of the low adhesion areas include firstapplying a bonding material and then removing the bonding material,where the bonding material is able to remove the coating from theaperture area but allow the surrounding coating to remain intact. Lowindex coatings can also be patterned to form areas where electricalconnections can be made to the die. See, for example, U.S. PatentPublication US 2003/0111667 A1 (Schubert), incorporated herein byreference.

Metal reflective layers can be applied by conventional processes, andpatterned as needed to provide an aperture and appropriate electricalisolation.

Turning now to FIG. 6, we see there an LED package 60 that utilizes atapered optical element 62 to couple light out of the LED die 12. Asdiscussed in connection with optical element 42 of FIG. 4, opticalelement 62 also has a compound construction, i.e., it comprises at leasttwo sections 64, 66 joined together. The sections have input surfaces 64a, 66 a, output surfaces 64 b, 66 b, and reflective side surfaces 64 c,64 d, 66 c, 66 d as shown. The tapered side surfaces of element 62redirect or collimate (at least partially) light from closely positionedLED emitting surface 12 a in a non-imaging way. With tapered element 62and other tapered elements disclosed herein, the side surfaces need notbe planar. They can be conical, curved (including parabolic) or anysuitable combination depending on the intended application and designconstraints. The disclosed taper elements can have the shape of elementsknown in the art as CPCs (“compound” parabolic concentrators).

It is desirable in many situations to form the optical tapered elementfrom high refractive index materials to reduce reflections at the LEDemitting surface 12 a over the aperture defined by input surface 64 a,so that light is more efficiently coupled out of, or extracted from, theLED die 12. It is also desirable in many situations to fabricate theoptical element using a material having high thermal conductivity andhigh thermal stability. In this way, the optical element can perform notonly an optical function but a thermal management function as well.Further thermal management benefits can be gained by thermally couplingsuch an optical element to a heat sink, as is described in more detailin co-filed and commonly assigned U.S. patent application “LED PackageWith Front Surface Heat Extractor” (Attorney Docket No. 60296US002,which is incorporated herein by reference in its entirety.

Unfortunately, transparent materials that have sufficiently highrefractive indices at the LED emission wavelength, e.g., greater thanabout 1.8, 2.0, or even 2.5, and/or that have thermal conductivitiesgreater than about 0.2 W/cm/K, tend to be expensive and/or difficult tofabricate. Some of the relatively few materials that have both highrefractive index and high thermal conductivity include diamond, siliconcarbide (SiC), and sapphire (Al₂O₃). These inorganic materials areexpensive, physically very hard, and difficult to shape and polish to anoptical grade finish. Silicon carbide in particular also exhibits a typeof defect called a micropipe, which can result in scattering of light.Silicon carbide is also electrically conductive, and as such may alsoprovide an electrical contact or circuit function. Scattering withinoptical tapered elements may be acceptable if the scattering is limitedto a position near the input end of the element. However, it would beexpensive and time consuming to make a tapered element with sufficientlength to efficiently couple light from an LED die. An additionalchallenge in making one-piece tapered elements is that the materialyield may be relatively low, and the form-factor may force the LED dieto be individually assembled with the tapered element. For thesereasons, it can be advantageous to divide the tapered element into atleast two sections, the sections being made of different opticalmaterials, to reduce manufacturing cost.

A first section desirably makes optical contact with the LED die, and ismade of a first optical material having a high refractive index(preferably about equal to the LED die refractive index at the emittingsurface), high thermal conductivity, and/or high thermal stability. Inthis regard, high thermal stability refers to materials having adecomposition temperature of about 600° C. or more.

A second section is joined to the first section and is made of a secondoptical material, which may have lower material costs and be more easilyfabricated than the first optical material. The second optical materialmay have a lower refractive index, lower thermal conductivity, or bothrelative to the first optical material. For example, the second opticalmaterial can comprise glasses, polymers, ceramics, ceramicnanoparticle-filled polymers, and other optically clear materials.Suitable glasses include those comprising oxides of lead, zirconium,titanium, and barium. The glasses can be made from compounds includingtitanates, zirconates, and stannates. Suitable ceramic nanoparticlesinclude zirconia, titania, zinc oxide, and zinc sulfide.

A third section composed of a third optical material can be joined tothe second section to further aid in coupling the LED light to theoutside environment. In one embodiment the refractive indices of thethree sections are arranged such that n₁>n₂>n₃ to minimize overallFresnel surface reflections associated with the tapered element.

Oversized lens elements, such as the upper portion of optical element 42shown in FIG. 4, can be advantageously placed or formed at the outputend of disclosed simple or compound tapered elements. Antireflectioncoatings can also be provided on the surface(s) of such lens elementsand/or on input and output surfaces of disclosed optical elements,including tapered or other collimating elements.

In an exemplary arrangement, the LED die 12 can comprise a 1 mm×1 mm GaNjunction on a 0.4 mm thick slab of SiC. The first section 64 of thetapered element 62 can be composed of SiC. The second section 66 can becomposed of LASF35, a non-absorbing, non-scattering high index glasshaving n=2.0. The width dimensions of the junction between the first andsecond sections and the output dimensions of the second section can beselected as desired to optimize total light output into the surroundingenvironment, of refractive index 1.0. The edges of the 0.4 mm thick SiCslab can be tapered at a 12 degree negative slope to completelyfrustrate TIR modes of light reflection at the side surfaces of the LEDdie. This slope can be tailored as desired, since the absorption andscattering within the LED junction and SiC slab will change theintegrated mode structure compared to a standard encapsulated LED. Forexample, it may be desirable to use a positive slope (where the width ofthe LED junction is less than the width of the SiC slab) in order todirect optical modes away from the absorbing junction. The SiC slab may,in this manner, be considered as part of the tapered element.

The first section 64 can be coupled to a thermal heat sink as mentionedpreviously. The second section 66 can be bonded to the first section 64using conventional bonding techniques. If a bonding material is used, itcan have a refractive index between the two optical materials beingjoined in order to reduce Fresnel reflections. Other useful bondingtechniques include wafer bonding techniques known in the semiconductorwafer bonding art. Useful semiconductor wafer bonding techniques includethose described in chapters 4 and 10 of the text “Semiconductor WaferBonding” by Q.-Y. Tong and U. Gösele (John Wiley & Sons, New York,1999). Wafer bonding methods described U.S. Pat. No. 5,915,193 (Tong etal.) and U.S. Pat. No. 6,563,133 (Tong) may also be used.

The LED package 70 shown in FIG. 7 utilizes a compound tapered element72 in which a first section 74, having an input surface 74 a connectedto a larger output surface 74 b by tapered reflective side walls, isencapsulated in a second section 76, which also has an input surface 76a (coextensive with output surface 74 b) and an even larger outputsurface 76 b. The output surface 76 a is curved to provide the compoundelement 72 with optical power useful for further collimation orfocusing. The tapered side surfaces of section 74 are shown with acoating 78 of low refractive index material to promote TIR at suchsurfaces. The material preferably has a refractive index lower than thatof first section 74, second section 76, and LED die 12. Such coating 78can also be applied to the portion of emitting surface 12 a not incontact with section 74, and/or to the side surfaces 12 c (see FIG. 1)of LED die 12. In constructing LED package 70, first section 74 can bebonded to (or simply placed upon) the desired aperture zone of emittingsurface 12 a, and a precursor liquid encapsulating material can bemetered out in sufficient quantity to encapsulate the LED die and thefirst section, followed by curing the precursor material to form thefinished second section 76. Suitable materials for this purpose includeconventional encapsulation formulations such as silicone or epoxymaterials. The package can also include a heat sink coupled to the sidesof first section 76 through coating 78. Even without such a heat sink,use of a high thermal conductivity first section of the tapered elementcan add significant thermal mass to the LED die, providing some benefitat least for pulsed operation using a modulating drive current.

Both simple tapered elements and compound tapered elements disclosedherein can be manufactured by conventional means, such as by fabricatingthe tapered components individually, bonding a first segment to the LEDdie, and then adding successive segments. Alternatively, simple andcompound tapered elements can be manufactured using precision abrasivetechniques disclosed in co-filed and commonly assigned U.S. patentapplication “Process For Manufacturing Optical And SemiconductorElements”, Attorney Docket No. 60203US002, and U.S. patent application“Process For Manufacturing A Light Emitting Array”, Attorney Docket No.60204US002, both of which are incorporated herein by reference in theirentirety. Briefly, a workpiece is prepared that contains one or morelayers of the desired optical materials. The workpiece can be in a largeformat, such as wafers or fiber segments. A precisely patterned abrasiveis then brought into contact with the workpiece so as to abrade channelsin the workpiece. When abrasion is complete, the channels define amultiplicity of protrusions, which can be in the form of simple orcompound tapered elements. The tapered elements can be removedindividually from the workpiece and bonded one-at-a-time to separate LEDdies, or an array of tapered elements can conveniently be bonded to anarray of LED dies.

When optical coupling elements whose input surfaces are smaller than theemitting surface of the LED die are used, it becomes possible toconsider coupling multiple such elements to different portions of thesame emitting surface.

Advantageously, such an approach can be used to reduce the quantity ofoptical material necessary to couple a given amount of light out of theLED die, by simply replacing a single optical taper element with aplurality of smaller ones. The difference in material usage can beparticularly important when dealing with expensive anddifficult-to-work-with materials such as diamond, SiC, and sapphire. Forexample, replacing a single optical tapered element with a 2×2 array ofsmaller optical tapered elements can reduce the required thickness forthe high index (first) optical material by a factor of more than 2, anda 3×3 array can reduce the required thickness by a factor of more than3. Surprisingly, even though light may not be efficiently emitted fromthe LED in places between the input surfaces of the optical elements,modeling shows that this approach still has a very high net extractionefficiency.

Another advantage of using multiple optical coupling elements such astapered elements is that gaps or spaces are formed between the elementsthat can be utilized for various purposes. For example, the gaps orspaces can be filled with high refractive index fluids, metal heatconductors, electrical conductors, thermal transport fluids, andcombinations thereof.

Modeling was performed on an LED package in which the LED die wasconstructed of SiC and an absorbing layer adjusted such that 30% of thelight generated within the LED die was emitted from the LED whenimmersed in a 1.52 refractive index medium. This is representative oftypical LED devices. The model used a 3×3 array of optical taperedelements coupled to the LED emitting surface as shown in the LED package80 of FIG. 8. The LED die 12′ shown there has angled side surfaces 12 c′and front emitting surface 12 a′, to which three of the optical taperedelements 82, 84, 86 are shown coupled at their input surfaces 82 a, 84a, 86 a respectively. Note the spaces or gaps 83, 85 formed between thesmaller optical elements. The output surfaces 82 b, 84 b, 86 b couple toan input surface 88 a of larger optical tapered element 88, which hasoutput surface 88 b. The model also used a hemispherical lens (notshown) that was oversized relative to taper element 88, with its flatsurface attached to output surface 88 b, the lens being made of BK7glass (n=1.52). The tapered element 88 was modeled as being composed ofLAS35 (n=about 2). The model then evaluated different optical materialsfor the smaller taper elements, and different materials for the ambientspace surrounding the LED die, including gaps 83, 85.

The calculated output power (e.g. in Watts) of the modeled LED packageis as follows as a function of the small tapered element opticalmaterial (designated “A” in the table) and the ambient material(designated “B” in the table) Optical material for “B” SiC LASF35 BK7Vacuum Optical SiC 0.821 0.814 0.775 0.754 material for LASF35 0.8260.771 0.701 0.665 “A” BK7 0.625 0.613 0.537 0.466

When these values are normalized to the power output of a system using asingle SiC tapered element in place of the 3×3 array of smallerelements, the following results are obtained: Optical material for “B”SiC LASF35 BK7 Vacuum Optical SiC 100% 99% 94% 92% material for LASF35101% 94% 85% 81% “A” BK7  76% 76% 65% 57%

These tables show that an optical tapered element does not have to beoptically coupled over the full area of the LED emitting surface toefficiently extract light. The tables also show that the ambient volumebetween the small taper elements can have a low refractive index withoutcausing a substantial reduction in extraction efficiency.

The ambient volume can be filled with a material to increase extractionefficiency. The filler material can be a fluid, an organic or inorganicpolymer, an inorganic particle-filled polymer, a salt, or a glass.Suitable inorganic particles include zirconia, titania, and zincsulfide. Suitable organic fluids include any that are stable at the LEDoperating temperature and to the light generated by the LED. In somecases, the fluid should also have a low electrical conductivity and ionconcentration. Suitable fluids include water, halogenated hydrocarbons,and aromatic and heterocyclic hydrocarbons. The filler material can alsoserve to bond the optical tapered elements to the LED die.

At least a portion of the space between the optical elements can havemetal applied to either distribute current to the LED die, or to removeheat from the LED die, or both. Since metals have measurable absorptionof light, it can be desirable to minimize absorptive losses. This can bedone by minimizing the contact area of the metal with the die, andreducing the optical coupling to the metal by introducing a lowrefractive index material between the metal and the die surface, theoptical element, or both. For example, the contact area can be patternedwith an array of metal contacts surrounded by low index material whichare in electrical conduct with an upper metal layer. See e.g. the '667Schubert publication referenced above. Suitable low index materialsinclude a gas or vacuum, fluorocarbons such as fluorinert, availablefrom 3M Company, St. Paul, Minn., water, and hydrocarbons. The metal canextend into a media surrounding the optical element where heat can beremoved.

Fluids can also be provided between the tapered elements to removeadditional heat. The array of optical tapered elements can be in asquare array (e.g. 2×2, 3×3, etc.), a rectangular array (e.g. 2×3, 2×4,etc.), or a hexagonal array. The individual optical tapered elements canbe square, rectangular, triangular, circular, or other desired shape incross-section at their input or output surfaces. The array can extendover the entire emitting surface of the LED, or beyond, or only over aportion thereof. The tapered elements can be attached to the LEDemitting surface with a low softening temperature solder glass, a softinorganic coating such as zinc sulfide, a high index fluid, a polymer, aceramic filled polymer, or by providing the optical elements and LEDwith very smooth and flat surfaces, and mechanically holding the dieagainst the input surfaces of the optical elements.

Another LED package 90 having multiple optical elements 92, 94 and apatterned low index layer 96 is depicted in FIG. 9. The patterned lowindex layer 96 includes two apertures as shown over which opticalelements 92, 94 are disposed in optical contact with emitting surface 12a of the LED die. Layer 96 is also in optical contact with LED dieemitting surface 12 a, as well as with LED die side surfaces 12 c. LEDpackage 90 further includes a metal contact 98 shown atop a portion oflow index layer 96. Although not shown in FIG. 9, patterned layer 96 isalso patterned in the vicinity of metal contact 98, and metal contact 98desirably extends through holes in the layer 96 to provide electricalcontact to LED die 12. A second electrical contact can be provided atanother location on the LED die depending on the chip design.

Glossary of Selected Terms

-   “Brightness”: the luminous output of an emitter or portion thereof    per unit area and per unit solid angle (steradian).-   “Light emitting diode” or “LED”: a diode that emits light, whether    visible, ultraviolet, or infrared. The term as used herein includes    incoherent (and usually inexpensive) epoxy-encased semiconductor    devices marketed as “LEDs”, whether of the conventional or    super-radiant variety.-   “LED die”: an LED in its most basic form, i.e., in the form of an    individual component or chip made by semiconductor wafer processing    procedures. The component or chip can include electrical contacts    suitable for application of power to energize the device. The    individual layers and other functional elements of the component or    chip are typically formed on the wafer scale, the finished wafer    finally being diced into individual piece parts to yield a    multiplicity of LED dies.

Various modifications and alterations of the invention will be apparentto those skilled in the art without departing from the spirit and scopeof the invention. It should be understood that the invention is notlimited to illustrative embodiments set forth herein.

1. A light source, comprising: an LED die having an emitting surface;and a collimating optical element having an input surface and an outputsurface, the input surface being in optical contact with at least aportion of the emitting surface; wherein the optical element comprises afirst portion that comprises the input surface and that is composed of afirst material; wherein the optical element comprises a second portionthat comprises the output end and that is composed of a second material;wherein the first material has a refractive index greater than that ofthe second material.
 2. The light source of claim 1, wherein the firstportion has a thermal conductivity greater than that of the secondmaterial.
 3. A light source, comprising: an LED die having an emittingsurface; and a collimating optical element having an input surface andan output surface, the input surface being in optical contact with atleast a portion of the emitting surface; wherein the optical elementcomprises a first portion that comprises the input surface and that iscomposed of a first material; wherein the optical element comprises asecond portion that comprises the output surface and that is composed ofa second material; wherein the first material has a thermal conductivitygreater than that of the second material.
 4. The light source of eitherclaim 1 or 3, wherein the first material is selected from the group ofdiamond, silicon carbide, and sapphire.
 5. The light source of eitherclaim 1 or 3, wherein the output surface is curved.
 6. The light sourceof either claim 1 or 3, wherein the output surface is larger than theinput surface.
 7. The light source of either claim 1 or 3, wherein theoptical element comprises at least one reflective side surface betweenthe input and output surfaces.
 8. The light source of claim 7, furtherincluding a reflection-enhancing coating disposed on the at least onereflective side surface.
 9. The light source of claim 8, wherein thereflection-enhancing coating comprises a material whose refractive indexis about 1.4 or less.